1. Field of the Invention
The present invention pertains to a process for the preparation of quasi-crystalline boehmites.
2. Description of the Prior Art
Alumina, alpha-monohydrates or boehmites and their dehydrated and or sintered forms are some of the most extensively used aluminum oxide-hydroxides materials. Some of the major commercial applications involve one or more forms of these materials and these are, for example, ceramics, abrasive materials, fire-retardants, adsorbents, catalysts, fillers in composites and so on. Also, a major portion of the commercial boehmite aluminas is used in catalytic applications such as refinery catalysts, catalyst for hydroprocessing hydrocarbon feeds, reforming catalysts, pollution control catalysts, cracking catalysts. The term xe2x80x9chydroprocessingxe2x80x9d in this context encompasses all processes in which a hydrocarbon feed is reacted with hydrogen at elevated temperature and elevated pressure. These processes include hydrodesulfurization, hydrodenitrogenation, hydrodemetallization, hydrodearomatization, hydro-isomerization, hydrodewaxing, hydrocracking, and hydrocracking under mild pressure conditions, which is commonly referred to as mild hydrocracking. These types of aluminas are also used as catalysts for specific chemical processes such as ethylene-oxide production, and methanol synthesis. Relatively newer commercial uses of boehmite type of aluminas or modified forms thereof involve the transformation of environmentally unfriendly chemical components such as chlorofluorohydrocarbons (CFC""s) and other undesirable pollutants. Boehmite alumina types are further used as catalytic material for the treatment of exhaust gases of gas turbines for reducing nitrogen oxide.
The main reason for the successful extensive and diversified use of these materials in such variety of commercial uses, is their ability and flexibility to be xe2x80x9ctailorxe2x80x9d made to products with a very wide range of physical-chemical and mechanical properties.
Some of the main properties which determine the suitability of commercial applications involving gas-solid phase interactions such as catalysts and adsorbents are pore volume, pore size distribution, pore texture, specific density, surface areas, density and type of active centers, basicity and acidity, crushing strength, abrasion properties, thermal and hydrothermal aging (sintering) and long term stability.
To a large extent, the desired properties of the alumina product can be obtained by selecting and carefully controlling certain parameters which usually involve: raw materials, impurities, precipitation or conversion process conditions, aging conditions and subsequent thermal treatments (calcination/steaming) and mechanical treatments.
Nevertheless, in spite of all this large and diversified existing know-how, this technology still develops and presents unlimited scientific and technological challenges both to the manufacturers and end-users for further developments of such alumina based materials.
The term, boehmite, is used in the industry to describe alumina hydrates which exhibit XRD patterns close to that of the aluminum oxide-hydroxide [AlO(OH)], naturally occurring boehmite or diaspore. Further, the general term, boehmite, usually is used to broadly describe a wide range of alumina hydrates which contain different amounts of water of hydration, have different surface areas, pore volumes, specific densities, and exhibit different thermal characteristics upon thermal treatments. Yet their XRD patterns, although they exhibit the characteristic boehmite [AlO(OH)] peaks, usually vary in their widths and can also shift in their location. The sharpness of the XRD peaks and their location have been used to indicate degree of crystallinity, crystal size, and amount of imperfections.
Broadly, there are two categories of boehmite aluminas. Category I, in general, contains boehmites which have been synthesized and/or aged at temperatures close to 100xc2x0 C. and most of the time under ambient atmospheric pressures. In the present specification, this type of boehmite is referred to as quasi-crystalline boehmites. The second category of boehmite consists of so-called microcrystalline boehmites.
In the state of the art, category I boehmites, quasi-crystalline boehmites, are referred to, interchangeably as: pseudo-boehmites, gelatinous boehmites or quasi-crystalline boehmites (QCB). Usually these QCB aluminas have very high surface areas, large pores and pore volumes, lower specific densities than microcrystalline boehmites, disperse easily in water or acids, they have smaller crystal sizes than microcrystalline boehmites, and contain a larger number of water molecules of hydration. The extent of hydration of the QCB can have a wide range of values, for example from about 1.4 up and about 2 moles of water per mole of AlO, intercalculated usually orderly or otherwize between the octahedral layers.
The DTG (differential thermographimetry) curves, which are the water release from the QCB materials as function of temperature, show that the major peak appears at much lower temperatures as compared to that of the much more crystalline boehmites.The XRD Patterns of QCBs show quite broad peaks and their half-widths are indicative of the crystal sizes as well as degree of crystal perfection.
The broadening of the widths at half-maximum intensities varies substantially and typical for the QCB""s could be from about 2xc2x0-6xc2x0 to 2xcex8. Further, as the amount of water intercalated into the QCB crystals is increased, the main (020) XRD reflection moves to lower 2 xcex8 values corresponding to greater d-spacings. Some typical, commercially available QCB""s are; Condea Pural (copyright), Catapal (copyright) and Versal(copyright) products.
The category II of the boehmites consists of microcrystalline boehmites (MCB), which are distinguished from the QCBs due to their high degree of crystallinity, relatively large crystal sizes, very low surface areas, and high densities. Contrary to the QCB""s the MCB""s show XRD patterns with higher peak intensities and very narrow half-peak line widths. This is due to the relatively small number of water molecules intercalated, large crystal sizes, higher degree of crystallization of the bulk material and to lesser amount of crystal imperfections present. Typically, the number of molecules of water intercalated can vary in the range from about 1 up to about 1.4 per mole of AlO. The main XRD reflection peaks (020) at half-length of maximum intensities have widths from about 1.5 down to about 0.1 degrees 2-theta (2xcex8). For the purpose of this specification we define quasi-crystalline boehmites to have 020 peak widths at half-length of the maximum intensity of 1.5 or greater than 1.5xc2x0. Boehmites having a (020) peak width at half-length of the maximum intensity smaller than 1.5 are considered microcrystalline boehmites.
A typical MCB commercially available product is Condea""s P-200 (copyright) grade of alumina. Overall, the basic, characteristic differences between the QCB and MCB types of boehmites involve variations in the following: 3-dimensional lattice order, sizes of the crystallites, amount of water intercalated between the octahedral layers and degree of crystal imperfections.
Regarding the commercial preparation of these boehmite aluminas, QCB""s are most commonly manufactured via processes involving: Neutralization of aluminum salts by alkalines, acidification of aluminate salts, hydrolysis of aluminum alkoxides, reaction of aluminum metal (amalgamated) with water and rehydration of amorphous rho-alumina obtained by calcining gibbsite. The MCB type of boehmite aluminas, in general are commercially produced with hydrothermal processes using temperatures usually above 150xc2x0 C. and autogeneous pressures. These processes usually involve hydrolysis of aluminum salts to form gelatinous aluminas, which are subsequently hydrothermally aged in an autoclave at elevated temperatures and pressures. This type of process is described in U.S. Pat. No. 3,357,791. Several variations of this basic process exist involving different starting aluminum sources, additions of acids or salts during the aging, and a wide range of process conditions.
MCB""s are also prepared using hydrothermal processing of gibbsite. Variations of these processes involve; addition of acids, alkaline and salts during the hydrothermal treatment, as well as the use of boehmite seeds to enhance the conversion of gibbsite to MCB. These types of processes are described in Alcoa""s U.S. Pat. No. 5,194,243, in U.S. Pat. No. 4,117,105 and in U.S. Pat. No. 4,797,139.
Nevertheless, whether pseudo-, quasi- or microcrystalline such boehmite materials are characterized by reflections in their powder X-ray. The ICDD contains entries for boehmite and confirms that reflections corresponding to the (020), (021) and (041) planes would be present. For copper radiation, such reflections would appear at 14, 28 and 38 degrees two theta. The various forms of boehmite would be distinguished by the relative intensity and width of the reflections. Various authors have considered the exact position of the reflections in terms of the extent of crystallinity. Nevertheless, lines close to the above positions would be indicative of the presence of one or more types of boehmite phases.
In the prior art, we find QCB containing metal ions which have been prepared by the hydrolysis of alumina isopropoxide with the co-precipitation of lanthanides as described in the paper by J. Medena, J. Catalysis, vol. 37, 91-(1975), and J. Wachowski et al., Materials Chemistry, vol. 37, 29-38 (1994). This process is conducted at a pH above 7.0. The products are pseudo-boehmite type aluminas with the occlusion of one or more lanthanide metal ions. These materials have been primarily used in high temperature commercial applications where the presence of such lanthanide metal ions in the pseudo-boehmite structure retards the transformation of the gamma-alumina to the alpha-alumina phase. Therefore, a stabilization of the gamma phase is obtained, retaining a higher surface area before it converts to the refractory lower surface area alpha-alumina. Specifically Wachowski et al. used the lanthanide ions (La, Ce, Pr, Nd, Sm) in quantities from 1% to 10% by weight which were calcined at temperatures in the range of 500xc2x0 C. to 1200xc2x0 C.
Also, EP-A1-0 597 738 describes the thermal stabilization of alumina by addition of lanthanum, optionally combined with neodymium. This material is prepared by aging rehydrateable alumina (i.e. flash calcined gibbsite) in a slurry at a pH between 8 and 12 with a lanthanum salt at a temperature between 70 and 110xc2x0 C., followed with thermal treatment at a temperature between 100 and 1000xc2x0 C.
Further, EP-A-0 130 835 describes a catalyst comprising a catalytically active metal supported on a lanthanum or neodymium-xcex2-Al2O3 carrier. Said carrier is obtained by precipitation of aluminum nitrate solution with ammonium hydroxide in the presence of a lanthanum, praseodymium or neodymium salt solution. As the precipitated amorphous material is directly washed with water and filtered, the alumina is not allowed under the usual conditions and certain pH, concentration and temperatures to age with time so that it crystallizes to a boehmite alumina structure.
The present invention is directed to an improved process for the preparation of quasi-crystalline boehmite. In this improved process a quasi-crystalline boehmite precursor is aged at a pH below 7, preferably under hydrothermal conditions.
Other objectives and embodiments of our invention encompass details about compositions, manufacturing steps, etc., all of which are hereinafter disclosed in the following discussion of each of the facets of the present invention.